An aromatic carbonate is useful as a raw material for, e.g., the production of an aromatic polycarbonate (whose utility as engineering plastics has been increasing in recent years) without using poisonous phosgene. With respect to the method for the production of an aromatic carbonate, a method for producing an aromatic carbonate or an aromatic carbonate mixture is known, in which a dialkyl carbonate, an alkyl aryl carbonate or a mixture thereof is used as a starting material and an aromatic monohydroxy compound, an alkyl aryl carbonate or a mixture thereof is used as a reactant, and in which a transesterification reaction is performed between the starting material and the reactant, thereby producing an aromatic carbonate or an aromatic carbonate mixture which corresponds to the starting material and the reactant.
However, since this type of transesterification is a reversible reaction in which, moreover, not only is the equilibrium biased toward the original system but the reaction rate is also low, the production of an aromatic carbonate by the above-mentioned method on a commercial scale is accompanied with great difficulties. To improve the above-mentioned method, several proposals have been made, most of which relate to the development of a catalyst for increasing the reaction rate. As a catalyst for use in the method for producing an alkyl aryl carbonate, a diaryl carbonate or a mixture thereof by reacting a dialkyl carbonate with an aromatic hydroxy compound, there have been proposed various metal-containing catalysts, which include for example, a Lewis acid, such as a transition metal halide, or compounds capable of forming a Lewis acid (see, for example, Patent Document 1), a tin compound, such as an organotin alkoxide or an organotin oxide (see, for example, Patent Document 2), salts and alkoxides of an alkali metal or an alkaline earth metal, and lead compounds (see, for example, Patent Document 3), complexes of a metal, such as copper, iron or zirconium (see, for example, Patent Document 4), titanic acid esters (see, for example, Patent Document 5), a mixture of a Lewis acid and a protonic acid (see, for example, Patent Document 6), a compound of Sc, Mo, Mn, Bi, Te or the like (see, for example, Patent Document 7), and ferric acetate (see, for example, Patent Document 8).
As a catalyst for use in the method for producing a diaryl carbonate by a same-species intermolecular transesterification, wherein an alkyl aryl carbonate is disproportionated to a dialkyl carbonate and a diaryl carbonate, there have been proposed various catalysts, which include for example, a Lewis acid and a transition metal compound which is capable of forming a Lewis acid (see, for example, Patent Document 9), a polymeric tin compound (see, for example, Patent Document 10), a compound represented by the formula R—X(═O)OH (wherein X is selected from Sn and Ti, and R is selected from monovalent hydrocarbon groups) (see, for example, Patent Document 11), a mixture of a Lewis acid and a protonic acid (see, for example, Patent Document 12), a lead catalyst (see, for example, Patent Document 13), a titanium or zirconium compound (see, for example, Patent Document 14), a tin compound (see, for example, Patent Document 15), and a compound of Sc, Mo, Mn, Bi, Te or the like (see, for example, Patent Document 7).
Another attempt for improving the yield of aromatic carbonates in these reactions consists in displacing the equilibrium in the direction of the desired product formation as much as possible, by modifying the mode of the reaction process. For example, there have been proposed a method in which by-produced methanol is distilled off together with an azeotrope forming agent by azeotropic distillation in the reaction of a dimethyl carbonate with phenol (see, for example, Patent Document 16), and a method in which by-produced methanol is removed by adsorbing the same onto a molecular sieve (see, for example, Patent Document 17).
Further, a method is known in which an apparatus comprising a reactor having provided on the top thereof a distillation column is employed in order to separate and distill off alcohols (by-produced in the course of the reaction) from a reaction mixture obtained in the reactor (see, for example, Patent Document 18).
As more preferred methods for producing an aromatic carbonate, the present inventors previously developed a method in which a dialkyl carbonate and an aromatic hydroxy compound are continuously fed to a continuous multi-stage distillation column to effect a continuous transesterification reaction in the distillation column, while continuously withdrawing a low boiling point reaction mixture containing a by-produced alcohol from an upper portion of the distillation column by distillation and continuously withdrawing a high boiling point reaction mixture containing a produced alkyl aryl carbonate from a lower portion of the distillation column (see, for example, Patent Document 19), and a method in which an alkyl aryl carbonate is continuously fed to a continuous multi-stage distillation column to effect a continuous transesterification reaction in the distillation column, while continuously withdrawing a low boiling point reaction mixture containing a by-produced dialkyl carbonate by distillation and continuously withdrawing a high boiling point reaction mixture containing a produced diaryl carbonate from a lower portion of the distillation column (see, for example, Patent Document 20). These methods for the first time realized efficient, continuous production of an aromatic carbonate. Thereafter, various methods for continuously producing an aromatic carbonate have further been developed, based on the above-mentioned methods developed by the present inventors. Examples of these methods include a method in which a catalytic transesterification reaction is performed in a column reactor (see, for example, Patent Document 21), a method which uses a plurality of reactors which are connected in series (see, for example, Patent Document 22), a method in which a bubble tower reactor is used (see, for example, Patent Document 23), and a method in which a vertically long reactor vessel is used (see, for example, Patent Document 24).
Also, there have been proposed methods for decreasing the amounts of impurities and/or by-products contained in an aromatic carbonate produced by any of the above-mentioned methods. For example, it is known that when an aromatic carbonate is produced by transesterification, high boiling point substances (each having a boiling point higher than that of the aromatic carbonate) are likely to be by-produced. For example, Patent Document 8/Patent Document 25 discloses that when diphenyl carbonate is produced by a transesterification of dimethyl carbonate with phenol, an impurity having a boiling point equal to or higher than the boiling point of the produced diphenyl carbonate is by-produced, and that the impurity is caused to enter the diphenyl carbonate and causes the discoloration of an ultimate product, such as an aromatic polycarbonate. This prior art document does not disclose an example of the impurity having a boiling point equal to or higher than the boiling point of the produced diphenyl carbonate; however, as an example of the impurity, there can be mentioned an aryloxycarbonyl-(hydroxy)-arene which is produced as an isomer of a diaryl carbonate by Fries rearrangement. More specifically, when diphenyl carbonate is produced as the diaryl carbonate, phenyl salicylate can be mentioned as an example of the aryloxycarbonyl-(hydroxy)-arene. Phenyl salicylate is a high boiling point substance whose boiling point is 4 to 5° C. higher than the boiling point of the diphenyl carbonate.
In this case, when the transesterification is conducted for a long period of time, the above-mentioned high boiling point substance accumulates in the reaction system and the amount of the impurity mixed into the product, namely an aromatic carbonate, tends to increase, so that the purity of the ultimate aromatic carbonate is lowered. Further, as the amount of the high boiling point substance in the reaction mixture increases, the boiling point of the reaction mixture rises, which in turn necessitates the elevation of the temperature of the reaction mixture so as to separate the high boiling point substance. As a result, the by-production of the high boiling point substance is accelerated, thus rendering it difficult to produce a desired aromatic carbonate stably for a prolonged period of time. As a measure for stably producing an aromatic carbonate for a prolonged period of time, there has been proposed a method in which a liquid reaction mixture containing a high boiling point substance and a metal-containing catalyst is withdrawn from the reaction system, followed by reacting the withdrawn reaction mixture with a specific reactant for separating the reaction mixture into a component derived from the high boiling point substance and a component derived from the metal-containing catalyst, thereby removing the high boiling substance from the reaction system (see, for example, Patent Document 26).
Further, impurities and/or by-products having boiling points lower than that of an aromatic carbonate are also known. Specifically, for example, Patent Document 27 proposes a method for separation of alkyl aromatic ethers (anisoles) from an aromatic carbonate.
However, heretofore, there has not been known any method which can be used for efficiently producing a high purity aromatic carbonate which exhibits advantageously high reactivity when used as a raw material for a transesterification aromatic polycarbonate, and, hence, it has been desired to develop such a method.
Patent Document 1: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 51-105032, Unexamined Japanese Patent Application Laid-Open Specification No. Sho 56-123948 and Unexamined Japanese Patent Application Laid-Open Specification No. Sho 56-123949 (corresponding to West German Patent Application Publication No. 2528412, British Patent No. 1499530 and U.S. Pat. No. 4,182,726)                Patent Document 2: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 54-48733 (corresponding to West German Patent Application Publication No. 2736062), Unexamined Japanese Patent Application Laid-Open Specification No. Sho 54-63023, Unexamined Japanese Patent Application Laid-Open Specification No. Sho 60-169444 (corresponding to U.S. Pat. No. 4,554,110), Unexamined Japanese Patent Application Laid-Open Specification No. Sho 60-169445 (corresponding to U.S. Pat. No. 4,552,704), Unexamined Japanese Patent Application Laid-Open Specification No. Sho 62-277345 and Unexamined Japanese Patent Application Laid-Open Specification No. Hei 1-265063        Patent Document 3: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 57-176932        Patent Document 4: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 57-183745        Patent Document 5: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 58-185536 (corresponding to U.S. Pat. No. 4,410,464)        Patent Document 6: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 60-173016 (corresponding to U.S. Pat. No. 4,609,501)        Patent Document 7: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 1-265064        Patent Document 8: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 61-172852        Patent Document 9: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 51-75044 (corresponding to West German Patent Application Publication No. 2552907 and U.S. Pat. No. 4,045,464)        Patent Document 10: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 60-169444 (corresponding to U.S. Pat. No. 4,554,110)        Patent Document 11: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 60-169445 (corresponding to U.S. Pat. No. 4,552,704)        Patent Document 12: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 60-173016 (corresponding to U.S. Pat. No. 4,609,501)        Patent Document 13: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 1-93560        Patent Document 14: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 1-265062        Patent Document 15: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 1-265063        Patent Document 16: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 54-48732 (corresponding to West German Patent Application Publication No. 2736063 and U.S. Pat. No. 4,252,737)        Patent Document 17: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 58-185536 (corresponding to U.S. Pat. No. 4,410,464)        Patent Document 18: Working examples of Unexamined Japanese Patent Application Laid-Open Specification No. Sho 56-123948 (corresponding to U.S. Pat. No. 4,182,726), Working examples of Unexamined Japanese Patent Application Laid-Open Specification No. Sho 56-25138, Working examples of Unexamined Japanese Patent Application Laid-Open Specification No. Sho 60-169444 (corresponding to U.S. Pat. No. 4,554,110), Working examples of Unexamined Japanese Patent Application Laid-Open Specification No. Sho 60-169445 (corresponding to U.S. Pat. No. 4,552,704), Working examples of Unexamined Japanese Patent Application Laid-Open Specification No. Sho 60-173016 (corresponding to U.S. Pat. No. 4,609,501), Working examples of Unexamined Japanese Patent Application Laid-Open Specification No. Sho 61-172852, Working examples of Unexamined Japanese Patent Application Laid-Open Specification No. Sho 61-291545, and Working examples of Unexamined Japanese Patent Application Laid-Open Specification No. Sho 62-277345        Patent Document 19: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 3-291257        Patent Document 20: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 4-9358        Patent Document 21: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 6-41022, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 6-157424 and Unexamined Japanese Patent Application Laid-Open Specification No. Hei 6-184058        Patent Document 22: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 6-234707 and Unexamined Japanese Patent Application Laid-Open Specification No. Hei 6-263694        Patent Document 23: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 6-298700        Patent Document 24: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 6-345697        Patent Document 25: Unexamined Japanese Patent Application Laid-Open Specification No. Sho 61-172852        Patent Document 26: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 11-92429 (corresponding European Patent No. 1016648 B1)        Patent Document 27: Unexamined Japanese Patent Application Laid-Open Specification No. Hei 9-176094        